Part passing FEA but failing lab test 4

[windsoreng]

I am designing brake pedal.

Its made of plastic, I am using CATIA/ANSYS for FEA analysis.

Part is within stress limits and sress counters are under yield stress, but during lab test, it fails.

I think that I am not modeling plastic correctly in FEA, I am using non linear model.

Any Ideas?

 

[KENAT]

Well, have you done any hand calcs to check your FEA is at least the right order of magnitude?

Have you verified the parts being tested are made correctly?

Have you verified the test is being conducted correctly?

You may want to ask the FEA question in the relevant software forum, or maybe in one of the structural analysis ones.

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[Pud]

The difference between manufacturer's data and "as moulded" figures can be very vast. As KENAT implied (amongst other things) - check the moulding conditions:

e.g.

Has the material been dried correctly (if required)?
Is the mould temperature high enough to attain maximum crystallinity?

Plus a myriad others. Best bet is to get material manufacturers on site to witness the moulding process.

H

http://www.tynevalleyplastics.co.uk

 

[Demon3]

I bet you are using a glass fiber filled plastic. You need to realise that those materials are only strong in one direction. The data you read from tensile tests are with the glass aligned in the best possible way. The modulus and yield strength in the other direction are only 20% of the value they quote.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

I agree with the above.

Non optimal moulding conditions.

Use of lab test piece data as opposed to real world design re load sharing.

Glass fibre orientation.

Part is not made to exact same dimensions as model, especially re sharp edges and radii.

A different grade may have been used for data production.

After moulding conditioning.

Data measured at a standard section thickness is not reliable at a different thickness due to changes in crystallinity at different depths into the plastic and different glass orientation at different thickness. This is sometimes called skin layer effect.

What material. What section thickness, what after treatments, what rib thickness and rad on ribs to beam junction.




Regards
Pat
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